1. Field of the Invention
The present invention concerns a monitoring method to monitor and/or protect modules, in particular a radio-frequency antenna of a magnetic resonance system, of the type wherein: a measurement step is implemented in which at least one first characteristic monitoring variable is measured by means of at least one first measurement unit; and at least one calculation step is implemented in which at least one maximum limit value for a power and/or a voltage within the module is calculated by means of the at least one first measured characteristic monitoring variable.
The present invention also concerns a monitoring device that implements such a method, and a magnetic resonance apparatus that includes such a monitoring device.
2. Description of the Prior Art
In magnetic resonance systems, due to the increasing utilization of existing, design-dependent limits and/or limit values of individual modules, it is necessary to provide monitoring to comply with the existing limits and/or limit values for the individual modules. Endangerment and/or destruction of the individual modules—in particular of a radio-frequency antenna of the magnetic resonance systems—can be reduced or prevented by this monitoring.
It is reasonable to monitor both occurring peak voltages and an average power of the radio-frequency antenna. For example, in the operation of the radio-frequency antenna peak voltages that are too high can lead to arcing and to the destruction of capacitors of the radio-frequency antenna in particular. For example, an average power that is too high can lead to a heating of individual modules of the radio-frequency antenna and therefore can contribute to an increased wear of the individual modules. In addition, due to its immediate proximity to a patient during a magnetic resonance measurement, the radio-frequency antenna and/or individual components of the radio-frequency antenna must comply with predetermined temperature limit values during a measurement operation in order to avoid endangering the patient.
In order to comply with existing limit values, it has been proposed to design the radio-frequency antenna and/or individual components and/or modules of the radio-frequency antenna so as to be markedly overdimensioned. In magnetic resonance systems fashioned in such a manner, the maximum average transmission power and/or the maximum peak power fall markedly below the limit values to be complied with.
It is also known to determine a reference voltage for a patient in order to protect the radio-frequency antenna. For the reference voltage, a magnetic resonance-active magnetic field of 11.75 μT is normally generated and the voltage associated therewith is measured. An allowable maximum transmission voltage is subsequently determined by means of the measured reference voltage, by multiplying the measured reference voltage with a pre-established scaling factor. However, this method has the disadvantage that, for example, a wiring error can lead to a radio-frequency magnetic field that—in comparison with a radio-frequency magnetic field with a correct wiring of the radio-frequency antenna—points in an opposite direction, so due to the wiring error a markedly higher transmission power is generated and/or a markedly higher voltage is present within the radio-frequency antenna than with a correct wiring. The measurement of the reference voltage takes place at measurement points arranged at a distance from the radio-frequency antenna so even though a markedly higher voltage can be present at the radio-frequency antenna, such a higher voltage is not detected by the measurement.
In addition to this, a load (for example a patient) and/or a phantom must always be located within the radio-frequency antenna for the reference measurement. Since the calculated allowable maximum transmission voltage is directly proportional to the measured reference voltage, the allowable maximum transmission voltage is thus subject to the same error sources that can occur for the measured reference voltage. An additional disadvantage of these methods is that an appropriate component protection—in particular the radio-frequency antenna of the magnetic resonance system—is only effective after the determination of the reference voltage, and thus the limit values for a maximum transmission power and/or an average maximum transmission power can already be exceeded in the determination of the reference voltage.